This invention relates to a process for finishing working surfaces of workpieces, and in particular to a controlled surface vibratory finishing process for providing superior finishes on exterior surfaces of workpieces constructed from many different hard materials such as, for example, steel or ceramic. This invention finds a wide range of application for any workpiece having one or more exterior surfaces. The exterior surfaces can be, for example, working surfaces against which moving components of the device are maintained in contact during operation or use of the device, including machine parts such as antifriction bearings and gears. As will be discussed below, the surface finishing process used in the present invention is any vibratory finishing process, including barrelling, tumbling, rotating, agitating, spinning, shaking or centrifugal processes, where one or more workpieces are placed in a container or similar device with an abrasive medial or abrading elements that displace portions of the workpiece during the vibratory finishing process. The vibratory finishing process can be performed with or without a solution in the container.
As is known in the art, conventional manufacturing processes for workpieces contructed from hard materials such as metals or ceramic generally are classified in one of three categories: (1) processes used primarily to change the shape of materials (e.g., casting, forging, machining, pressing); (2) processes used to finish parts to a desired dimension (e.g., grinding, hobbing, drilling, shaping, milling, torch cutting); and (3) processes used primarily to obtain a desired surface finish (e.g., polishing, honing, superfinishing, painting, lapping, electroplating). The types of processes used to produce a particular product or workpiece are based upon factors such as, for example, volume of production, quality of the finished product, cost, and efficiency. Most workpieces can be produced by several methods, and considerable resources are devoted to developing one manufacturing process that is most economical.
Some products are suitably finished for commercial use after undergoing the first step set forth above in the manufacturing process. However, other products require further processing to obtain the desired dimensions and surface finish. For example, specialty products and machine parts such as bearings or toothed gears are finished to within close tolerances with a final machining on their critical working surfaces (i.e., surfaces against which major components of the machine part are maintained in contact during operation of the device). Certain products and machine parts are subjected to one of the surface finishing processes set forth above to provide an enhanced finish along their working surfaces to enable the parts to operate at lower torques and with less heat generation. Such critical working surfaces initially are finished with a finish grinding or turning, and finally a honing or superfinishing to provide the enhanced finish.
However, the critical surfaces with enhanced finishes may have directional surface textures and surface irregularities which degrade the performance of the parts if the surface finishing process is terminated prematurely or if the selected finishing process is not a preferred production method for that product. Furthermore, the surface finishing process may destroy the profile of the exterior surface of the workpiece is the processing time continues for an extended time period after the desired surface finish is obtained. Since a trial and error approach is used in the prior art to determine the time necessary to obtain the desired surface finish, the third manufacturing process is often time consuming, inefficient and not cost-effective. Nonetheless, such surface finishing processes are necessary and widely used to provide workpieces that perform properly for a given application.
To overcome the undesirable effects associated with enhanced finishing of workpieces, vibratory finishing processes are used in certain situations to finish exterior surfaces. Vibratory finishing processes, including barrelling, tumbling, rotating, agitating, spinning, shaking or centrifugal processes, are used to provide a desired surface roughness on the exterior surfaces. In certain applications, vibratory finishing is used to obtain an isotropic surface. Surface irregularities associated with isotropic surfaces have no orientation, resulting in a superior surface when compared to those receiving enhanced finishes. In other applications, an isotropic surface is not needed or desired, and the vibratory finishing process can be stopped before an isotropic surface is obtained. As discussed in U.S. Pat. No. 5,503,481, issued Apr. 2, 1996 to F. Hashimoto et al. and assigned to The Timken Company, the disclosure of which is incorporated herein by reference, vibratory finishing processes provide superior working surfaces for bearing components. Many advantages of vibratory finishing are reported in the following articles: A. P. Babichev, The Vibratory Polishing of Turbine Blades, Machine & Tooling, 1964, 35: 41-44; R. G. Dargis, Chemical Accelerated-Vibratory Deburring of Steel Parts, SME, 1984, MR84-943; L. K. Gillespie, A Quantitative Approach to Vibratory Deburring Effectiveness, SME, 1975, MRR75-11; and W. H. Safrenek, A. C. Secrest, and J. C. Turn, Chemical Accelerators for Vibratory Finishing, Manufacturing Engineering, November 1976, pp. 32-33.
However, as discussed above with respect to the other surface finishing processes, a trial and error approach is employed in the prior art to determine the processing time for exposure of the workpiece to the vibratory finishing process to achieve the desired surface roughness or an isotropic finish. If the workpiece remains in certain vibrating solutions too long, the profiles of the working surfaces degrade. Furthermore, the trial and error method is inefficient and not preferred, since the workpieces often remain in the vibrating tub after the desired surface roughness is obtained, or the surface has become isotropic. Time, resources and expense are wasted when the vibratory finishing process continues beyond the time necessary to achieve the desired surface roughness or isotropic surfaces.
The systematical fundamentals of vibratory finishing have not been established prior to the present invention. A universal method for vibratory finishing that produces a desired surface roughness or an isotropic surface on an exterior surface of a workpiece without degrading the profile has not been determined. Furthermore, no common procedures exist to evaluate the vibratory finishing system, including the process set-up conditions, equipment, media, solution, etc. To apply the vibratory finishing technology to the final finishing process for workpieces such as specialty products, it is necessary to develop a process based upon control of process parameters, such as surface roughness and stock removal.
The present invention resides in an improved process founded upon formulas and mathematical modeling for a vibratory finishing process that provides for determination of process parameters, such as an optimum processing time to achieve a desired or aimed surface roughness based upon an initial surface roughness of a workpiece. Based upon the process parameters, superior exterior surfaces for the workpiece are provided in a minimum processing time. The validity of the modeling is illustrated by comparison to experimental results. The invention also resides in workpiece surfaces having a desired or aimed surface roughness after being exposed to the vibratory finishing process for a predetermined optimum processing time.